1. Field of the Invention
This invention relates to a ring oscillator having a plurality of inverting circuits connected in a ring through which a pulse edge is circulated. This invention also relates to a pulse phase difference encoding circuit which uses such a ring oscillator in encoding the phase difference between two pulse signals into a digital signal.
2. Description of the Prior Art
U.S. Pat. No. 5,128,624 corresponding to Japanese published unexamined patent application 3-220814 discloses a pulse phase difference encoding circuit which detects the phase difference between two pulse signals and then encodes the detected phase difference into a digital signal.
The pulse phase difference encoding circuit in U.S. Pat. No. 5,128,624 uses a ring oscillator having a given odd number of inverting circuits connected in a ring through which a pulse edge is circulated. When a first pulse signal PA is inputted into the pulse phase difference encoding circuit, the pulse edge starts to be circulated in the ring oscillator. The pulse phase difference encoding circuit includes first and second detecting sections responsive to a second pulse signal PB which is inputted into the pulse phase difference encoding circuit after the first pulse signal PA is inputted. The first section detects the number of rounds of the pulse edge, that is, the number of times of complete circulation of the pulse edge, in the ring oscillator during a period equal to the phase difference between the two pulse signals PA and PB. The second section detects an inverting circuit in the ring oscillator which the pulse edge reaches at the moment of the inputting of the pulse signal PB. In other words, the second section detects the position of the pulse edge in the ring oscillator at the moment of the inputting of the pulse signal PB. The first and second sections convert the detected parameters into corresponding digital signals which are combined into a composite digital signal representative of the phase difference between the pulse signals PA and PB.